1. Field of the Invention
The present invention relates to an organic light emitting display device, and more particularly to an organic light emitting display device decreasing the perception of a dark pixel occurring due to a short-circuit between a first electrode and a second electrode of an organic light emitting diode.
2. Discussion of Related Art
Organic light emitting display devices are spontaneous emission devices that emit light by re-combination of electrons supplied from a cathode and holes supplied from an anode. An electroluminescent (EL) display using the organic light emitting display device does not require additional back light, has a wider angle of visibility, higher response speed compared with a passive EL device, a lower direct current drive voltage, and can be formed in a ultra-thin pattern. Therefore, it may be implemented in a wall hanging type of display or a portable display.
The organic light emitting display device is driven either by a passive matrix method or by an active matrix method using a thin film transistor. In a display driven according to the passive matrix method, an anode and a cathode are formed to intersect, and a line is selected to be driven. In a display driven according to the active matrix method, a thin film transistor is connected to each anode electrode (indium tin oxide (ITO)) and is driven by a voltage maintained by a capacitor, which is connected to a gate of the thin film transistor.
FIG. 1 is a block diagram showing a conventional organic light emitting display device. The conventional organic light emitting display device includes a display region 10, a data driver 20, and a scan driver 30.
The display region 10 includes a plurality of data lines D1, D2, D3 . . . Dm, and a plurality of scan lines S1, S2, S3 . . . Sn, arranged to cross directions, and a plurality of pixels 11. The data lines D1, D2, D3 . . . Dm carry a data signal, and the scan lines S1, S2, S3 . . . Sn carry a scan signal. The pixels 11 are formed at intersections of the data lines D1, D2, D3 . . . Dm and the scan lines S1, S2, S3 . . . Sn.
The data driver 20 outputs a data signal indicating an image signal through the data lines D1, D2, D3 . . . Dm. The scan driver 30 sequentially outputs a select signal through the scan lines S1, S2, S3 . . . Sn to drive the pixel 11. The pixel 11 may include sub-pixels.
FIG. 2 is a circuit diagram showing one unit pixel of the conventional organic light emitting display device, and shows a representative pixel among n×m pixels of the display region in the organic light emitting display device shown in FIG. 1.
As shown in FIG. 2, the circuit of pixel 11 includes an organic light emitting diode OLED, a switching transistor M1, a drive transistor M2, a capacitor Cst, a scan line Sn, and a data line Dm. When the pixel 11 includes sub-pixels, each sub-pixel would include a similar circuit.
A gate of the switching transistor M1 is connected to the scan line Sn, and a source thereof is connected to the data line Dm. The switching transistor M1 transfers a data signal from the data line Dm to a gate of the drive transistor M2 in response to a select signal from the scan line Sn. A source of the drive transistor M2 is connected to a power source voltage ELVDD, and a capacitor Cst is connected between a gate and the source of the drive transistor M2. The capacitor Cst maintains the gate-source voltage Vgs of the drive transistor M2 during a predetermined time period.
A cathode b of the organic light emitting diode OLED is connected to a reference voltage ELVSS. The organic light emitting diode OLED emits light according to an electric current applied through the drive transistor M2. The reference voltage ELVSS connected to the cathode b of the organic light emitting diode OLED is less than the power source voltage ELVDD, and a ground voltage can be used as the reference voltage ELVSS.
The electric current flowing through the organic light emitting diode OLED is expressed by a following equation 1:
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    gs                                    -                                      V                    th                                                  )                            2                                =                                    β              2                        ⁢                                          (                                                      V                    DD                                    -                                      V                    data                                    -                                                                                V                      th                                                                                          )                            2                                                          (        1        )            where, IOLED is the electric current flowing through the organic light emitting diode OLED, Vgs is a voltage between a gate and a source of the drive transistor M2, Vth is a threshold voltage of the drive transistor M2, Vdata is a data voltage, and β is a constant.
As indicated in the equation 1, according to the pixel circuit shown in FIG. 2, an electric current corresponding to applied data voltage Vdata is supplied to the organic light emitting diode OLED, so that the organic light emitting diode OLED emits light corresponding to the applied data voltage Vdata.
FIG. 3 is a plan view showing one unit pixel of the conventional organic light emitting display device. This plan view may also correspond to a sub-pixel within a pixel.
The conventional unit pixel includes a scan line 32 arranged along one direction, a data line 31 arranged along a direction intersecting the direction of the scan line 32, and a power supply line 37 arranged parallel with the data line 31 to intersect the scan line 32. Furthermore, the switching transistor 33 is connected to the scan line 32 and the data line 31, respectively. A capacitor includes a lower electrode 35 and an upper electrode 36. The lower electrode 35 is connected to one of source/drain electrodes 34 of the switching transistor 33 through a contact hole. The upper electrode 36 is connected to the power supply line 37 and is arranged at an upper side of the lower electrode 35 of the capacitor. A gate 38 of the drive transistor 39 is connected to the lower electrode 35 of the capacitor. The drive transistor 39 includes a source/drain electrode 40 that is connected to an anode electrode a through a via 41.
In the conventional unit pixel, the organic light emitting diode OLED includes the anode electrode a, an organic emission layer, and a cathode electrode b. The anode electrode a is formed on a substrate. The organic emission layer is formed over an upper surface of the anode electrode a. The cathode electrode b is formed over an upper surface of the organic emission layer. The cathode electrode b and the organic emission layer are not shown in FIG. 3.
Furthermore, only the organic emission layer (shown in FIG. 4) exists between the anode electrode a and the cathode electrode b. An insulation film exists around the anode electrode. This prevents the anode electrode a and the cathode electrode b from electrically conducting to each other without the current first passing through the organic emission layer.
However, in the conventional organic light emitting diode OLED, one anode electrode and one cathode electrode are arranged in one unit pixel. During the manufacturing process, minute dust is interposed between the anode electrode and the cathode electrode. Due to patterning flaws and external pressure, the anode electrode and the cathode electrode that are to be insulated from each other, may contact and conduct, thereby causing a short. This is shown in FIG. 4.
FIG. 4 is a cross-sectional view showing a short-circuit between an anode electrode and a cathode electrode of a conventional organic light emitting display device. Reference numeral a represents the anode electrode, reference numeral b represents the cathode electrode, and reference numeral c represents minute dust.
As shown in FIG. 4, because the minute dust c penetrates the insulation film between the anode electrode a and the cathode electrode b, the anode electrode a and the cathode electrode b are short-circuited. Due to the short-circuit between the anode electrode a and the cathode electrode b, a cathode voltage ELVSS is applied to the anode electrode a. Accordingly, the drain current of the drive transistor, that corresponds to the data signal, flows into the shorted cathode electrode b instead of into the organic emission layer, thereby not emitting the intended color. This causes a dark pixel to be displayed and deteriorates the image.
Typically, a unit pixel includes a plurality of sub-pixels. Each of the sub-pixels includes an organic film having a different material and thickness according to a color to be embodied. Many dark pixels due to a short-circuit between the anode electrode a and the cathode electrode b may occur at a sub-pixel formed by a thin organic emission layer. According to experimental results, the occurrence rate of a progressive dark pixel in a blue sub-pixel is more than 10 times other colors. An improved scheme is therefore desirable.